Automobiles are generally classified as two types depending on whether the power transmission is manual or automatic. Soft copper wires are predominantly used as electrical conductors in an automotive wire harness. Because automobiles with an automatic transmission system are gaining wider acceptance today, there has been a shift from use of a carburetor to an electronic fuel injection system and a corresponding increase in the number of electronic instruments and other devices aboard vehicles. As a result, the number of electric and electronic wiring circuits in an automobile has increased so markedly that an increase not only in the space of the automobile occupied by the wire harness but also in the vehicle harness weight has occurred. From the viewpoint of fuel economy, the vehicle weight is desirably as light as possible and the increase in the volume of the automotive wire harness is not consistent with this objective. Hence, a need has arisen to reduce the automotive harness weight and space for the principal purpose of reducing the vehicle weight.
Theoretically, a very thin wire such as a lead will suffice for use in small-current circuits such as those including micro-computers in an automotive harness. In practice, however, the vibrational impact that develops while the car is running is so great that, in the absence of high mechanical strength, disconnection of the joints or wire breakage might occur to impede smooth running of the car. Therefore, in order to insure sufficient mechanical strength, it has been necessary to use conductors thicker than the diameter theoretically required in electrical terms.
To realize lighter electric wires, hard copper wires that are capable of insuring mechanical strength with small conductor diameter have been considered. However, the elongation of hard copper is so small that even if two terminals of hard copper wires are joined by thermocompression, the joint may be damaged under an externally exerted mechanical load. Thus, the area at which the terminals are thermocompressed becomes a mechanical weak point, which will readily break upon external impact and hence has low reliability.
The automotive harness weight could be reduced by employing smaller-diameter conductors but with conventional soft copper wires, the outside diameter of a conductor cannot be reduced without loss of mechanical strength. Under these circumstances, Cu-Sn alloys, Cu-Fe-P alloys useful as lead materials, Cu-Fe-P-Ni-Sn alloys, etc. have been designed as copper alloys that have high strength, improved cyclic bending strength and good electric conductivity and which, as a result, insure the production of conductors having satisfactory mechanical strength even if their outside diameter is reduced.
As shown in JP-B-60-30043 (the term "JP-B" as used herein means an "examined Japanese patent publication"), Cu-Sn alloys have satisfactory elongation and cyclic bending strength. Although their tensile strength is improved by forming a solid solution of Sn, the improvement is still insufficient. Another disadvantage of Cu-Sn alloys is their low conductivity. Cu-Fe-P alloys are designed to provide improved conductivity and tensile strength by dispersing and/or precipitating an Fe-P compound therein. However, the elongation and cyclic bending strength of Cu-Fe-P alloys are too small to justify their use as conductor materials. Cu-Fe-P-Ni-Sn alloys are intended to provide improved tensile strength by dispersing and/or precipitating an Fe-P compound and by forming a solid solution of Sn. Although Cu-Fe-P-Ni-Sn alloys have excellent elongation and cyclic bending strength, they have the disadvantage that Sn is dissolved in such a great amount that a marked drop in electric conductivity occurs.